In an optical transmission system, in particular in a transmission system based on the technique of wavelength multiplexing, it is increasingly required to monitor the following important optical transmission-related parameters:                the medium wavelength,        the relative channel length,        the optical transmission level, and        the optical signal noise ratio.        
It is known to measure the optical transmission-related parameters when drafting and installing a transmission line. In this context, in most cases, an optical spectral analyser with an accordingly adapted interpretation software is used to determine the above mentioned transmission-related parameters. However, the measurements are carried out when installing the line represent only momentary measuring. Changes in the individual network components, for example, those caused by their ageing or by replacing the original network components with new network components, are not registered by this momentary measuring.
It is further known to monitor the transmission system by using a test signal, said test signal having a wavelength beyond the wavelength range that is usually used for the transmission of the electrical and, respectively, optical signals.
It is further known to register one part of the above mentioned parameters continuously by means of an optical channel monitor module and to forward it to a central administration system. A value that is difficult to measure in this context is the so-called medium wavelength or, respectively, the relative channel position in a wavelength multiplexer system. Under medium wavelength is to be understood in this context the wavelength which indicates the centre of a wavelength range (wavelength interval) covered by a channel.
For its (the medium wavelength) measuring, a broadband part of the light representing the respective signal is usually first decoupled from the transmission channel. The broadband part is then split into its spectral parts via a wavelength-dependent element, for example, via an optical filter, and the spectral elements are then analysed.
The methods used to accomplish this are known, and are the use of a Fabry-Perot interferometer, a so-called Bragg grating or wavelength dependent attenuators. U.S. Pat. No. 5,986,782 describes a device for monitoring signal-to-noise in WDM optical communication systems. J. Yoshuda et al., Optoelectronic and Photonic Integrated Devices for Optical Modules in Fiber Optic Subscriber Systems”, presented at “Discovering A New World Of Communications”. Chicago, June 14-18, 1992; Vol. 3, Proceedings Of The International Conference On Communications, (New York, IEEE 1992), pages 1784-1788, describes various devices that can be used in optical communications. The disadvantage of the known methods is especially that only the power of the individual signal parts with the corresponding wavelengths can be determined.
The problem is solved by the present invention and is based on the use of an optical channel monitor chip with which it is easier to monitor the individual medium wavelengths of the signal parts. The problem is solved by the optical channel monitor chip with the features as described herein in the specification and the claims.